Roof Ventilation
One square foot of vent for every 150 square feet of attic. That is the code baseline, and almost nobody quotes it, because the number people remember is 1:300 — which is not the standard at all. It is an exception you have to earn.
The rule, and the exception
The minimum net free ventilating area is 1/150 of the vented space. You may halve that to 1/300 by putting between 40 and 50 percent of the required area high in the attic — within three feet of the ridge — with the balance down at the eaves. In the coldest climate zones a Class I or II vapor retarder on the warm-in-winter side of the ceiling also qualifies. Note what the exception is really rewarding: not fewer vents, but vents in the right two places.
Net free area is not hole size
A vent's net free area is what is left after the screen, louvers and baffles are subtracted, and it is published for every product. A ten-by-ten-inch louvered vent is nothing like a hundred square inches of airflow. This is where the arithmetic usually goes wrong, on paper, before anyone climbs a ladder.
Intake and exhaust have to match
Ventilation is a balance problem, not a parts problem. Ridge exhaust only works if soffit intake can feed it; add exhaust without intake and the roof pulls its makeup air out of the house instead. The single most common fault anyone finds up there is soffit vents buried under insulation pushed into the eaves — the vents are present, installed, and doing nothing. Mixing exhaust types on one attic space, a ridge vent competing with a powered unit, defeats both.
What poor airflow does
Trapped heat bakes shingles from beneath, softening the asphalt and driving off the granules that protect it. Trapped moisture is worse and quieter: warm indoor air carries water vapor up, it condenses on cold sheathing and nail points, and it drips. Owners find damp insulation, dark staining under the deck, rusted fasteners and eventually rot — then mistake all of it for a roof leak, because it looks exactly like one.
Ice dams are a heat problem, not a snow problem
It takes only a couple of inches of snow on the roof and a warm attic underneath. Heat escaping into the attic melts snow on the upper slope, the meltwater runs down to the cold eave beyond the heated space, refreezes there, and the ridge of ice that builds up pushes the next water backward under the courses. More insulation and better airflow fix it; a bigger gutter does not. Freezing rain is a different hazard entirely — that is clear ice arriving straight from the sky and loading the gutters and edges with weight, and no amount of attic ventilation changes it.
Fix it while the roof is open
A new ridge cut, added intake, or reworked eave details are straightforward with the shingles off and awkward afterward. Add baffles so insulation cannot choke the soffits, seal the attic bypasses around light fixtures and top plates so household moisture stops arriving in the first place, and duct bathroom and kitchen fans all the way outside rather than into the attic. Customers describe attic work added during a replacement as the part they were gladdest to have done at the time rather than later.
